I have yet to see a wi-fi signal scanner in the App Store. I suspect that Apple does not give developers the necessary access to the hardware this type of app. One developer, in the app description, even goes so far as to say Apple prohibits this kind of app.
It's one of the few app types I miss from my old Droid.
Hmm, looking again I do see one. It's called WiFi Radar. It sounds very limited and has horrible reviews. From the description it only lets you know there is a network around, not any info about it. You can get that much by going to the iPad's Network settings.